1. Field of the Invention
The present invention relates to a motor driving device for controlling a driving state of a motor used in a fail-safe designed system, and to a method of judging a state of the motor driving device.
2. Description of Related Art
It is known to design a system in fail-safe structure so that the system always operates on the safe side when there occurs any fault in the components constituting the system. As such a system (may be referred to as “fatal system” hereinafter), there are a pre-crash safety system, and an electric power steering system, for example.
Each of the pre-crash safety system and the electric power steering system includes a motor to drive a certain member, and a motor driving device which controls a driving state of the motor. Generally, the motor driving device includes a driver circuit to supply power to the motor, a current detecting circuit to detect a value of a current flowing through the motor, an over-current detecting circuit to detect an over-current in the motor and output a stop signal when an over-current is detected, and a microcomputer to control the driver circuit in accordance with output signals from the current detecting circuit and the over-current detecting circuit.
Such a motor driving device, which is required to have high reliability because it is used in a fatal system, is configured to perform a state inspection to make a judgment whether or not there is any fault therein when it starts to operate. It is desirable that the state inspection is performed while the motor is actually driving a driven member in terms of accuracy of the state inspection. However, in this case, since the driven member moves at a timing which is not anticipated by the user of the system, the user may have a feeling of concern.
Accordingly, there has been developed a motor driving device which includes an additional circuit (or additional signal path) for enabling the microcomputer thereof to apply false signals to the driver circuit and the over-current detecting circuit thereof in order to bring them in the same state as the starting state of the motor without actually driving the motor. In this motor driving device provided with the additional circuit, each of its components such as the driver circuit and the current detecting circuit is subjected to the state inspection individually one by one. For more details, refer to Japanese Patent Application Laid-open No. 11-314573, for example.
However, this motor driving device has problems in that its manufacturing cost and size are large because of the provision of the additional circuit, and that even when there occurs a fault only in the additional circuit, it is judged that there is a fault in at least one of the components of the motor driving device.